1. Field of the Invention
This invention relates to electrical circuit overcurrent protection.
2. Introduction to the Invention
Mechanical switches are widely used to control the flow of current in electrical circuits. The term "mechanical switch" is used herein to denote an electrical switch which comprises mechanical contacts which open or close in response to a mechanical (including manual), electrical, thermal or other form of activation. Such devices include simple manual switches, circuit breakers, Ground Fault Interrupts (GFIs), relays and bimetal devices (also referred to as electrothermal relays, thermally activated switches and electro-thermal devices). GFI's compare the current flowing at two different locations in a circuit, and interrupt the circuit if the currents differ by more than a predetermined amount, e.g. as the result of a ground fault between the locations. They do not, however, protect against faults which do not result in such a current imbalance, e.g. an overcurrent resulting from a short within the load which does not go to ground.
When a mechanical switch is operated to interrupt current flowing through it, arcing nearly always occurs between the contacts as they separate, even under normal operating conditions, and current (in the form of an arc) continues to flow through the switch until the arc is extinguished. The arc will damage the contacts to an extent which depends upon the current, the voltage, whether the current is AC or DC, the speed at which the contacts operate, and the material of which the contacts are made. A mechanical switch is usually rated according to the maximum current that it can safely interrupt at a stated AC or DC voltage and for a stated number of operations.
Arcing across contacts opening under high current conditions can cause such contacts to burn and result in catastrophic failure of the mechanical device. With such limitations in mind, attempts have been made to devise circuit configurations which limit the current flowing through the mechanical contacts, or limit the voltage across the mechanical contacts, or limit both the current and the voltage, as the mechanical contacts open, in order to protect the circuit protection switches and thereby protect the electrical circuits.
PTC circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions is in a low temperature, low resistance state. However, if the current through the PTC device increases excessively, and/or the ambient temperature around the PTC device increases excessively, and/or the normal operating current is maintained for more than the normal operating time, then the PTC device will be "tripped," i.e. converted to a high temperature, high resistance state such that the current is reduced substantially. Generally, the PTC device will remain in the tripped state, even if the current and/or temperature return to their normal levels, until the PTC device has been disconnected from the power source and allowed to cool. Particularly useful PTC devices contain a PTC element which is composed of a PTC conductive polymer, i.e. a composition which comprises (1) an organic polymer, and (2) dispersed, or otherwise distributed, in the polymer, a particulate conductive filler, preferably carbon black. PTC conductive polymers and devices containing them are described, for example in U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,317,027, 4,426,633, 4,545,926, 4,689,475, 4,724,417, 4,774,024, 4,780,598, 4,800,253, 4,845,838, 4,857,880, 4,859,836, 4,907,340, 4,924,074, 4,935,156, 4,967,176, 5,049,850, 5,089,801 and 5,378,407, the disclosures of which are incorporated herein by reference for all purposes.
In a batch of PTC devices made by the same manufacturing process, uncontrollable variations in the process can cause substantial variation in the conditions which will trip any individual device. The largest steady state current which will not cause any of the devices in the batch to trip is referred to herein as the "pass current" (I.sub.PASS) or "hold current", and the smallest steady state current which will cause all of the devices to trip is referred to as the "trip current" (I.sub.TRIP). In general, the difference between I.sub.PASS and I.sub.TRIP decreases slowly as the ambient temperature increases. Depending on the particular type of device, I.sub.TRIP may for example be 1.5 to 2.5 times I.sub.PASS at 20.degree. C. For any individual device, the pass current and the trip current are the same. However, in this specification, reference is made to a PTC device having an I.sub.PASS and a different I.sub.TRIP, because as a practical matter, the manufacturer of an electrical switch must make use of PTC devices taken from a batch of such devices. Generally, the higher the ambient temperature, the lower the pass current and the trip current. This phenomenon is referred to as "thermal derating", and the term "derating curve" is used to denote a graph of temperature against pass current.